I don't know if anyone's into mystery games, but I love them and my favorite one is freeware and is called Laura Bow and the Dagger of Amon Ra. It's an oldie, but a goodie. Sadly, I don't have any screenshots but here's the link to wikipedia if you wanna check it out. There are multiple ones with the same character, but this one is the best by far.

Note: by default, the resolution is set to a minimum so you have to change it yourself by joining a server and than hit the escape button (I hope they will make the options menu immediately accessible in next version (1.2)).

That link is for the demo. Furthermore Arx Fatalis is not entirely free (or open source). From the source code README file:

Prerequisites
-------------
You need a commercial installation of Arx to use these source files. Output binaries files (.exe, .dll) should target the game/ folder. Arx.exe will not launch unless you previously copied all Arx Fatalis content from install folder into the game/ folder.

Hi guys!
I have played this game a lot two years ago and found it interesting to share it in the net.
The game is much more updated than long time ago, so even the ones that have played
Transcendence might find smth new
Hope you enjoy:http://neurohack.com/transcendence/

As of June 23rd, Team Fortress 2 (by Valve Corporation) is now "Free To Play" on Steam. You can still buy in-game items via microtransactions, but the F2P version lets you enjoy the game just fine. I have played this game since Feb. 2008 and have just surpassed 2600 hours of gameplay.